The present invention relates to means for holding X-ray films, especially to cassettes or analogous arrangements for holding sheet-like X-ray films at the exposure station. More particularly, the invention relates to improvements in arrangements which serve to hold X-ray films between two intensifier screens by means of magnetic and magnetizable layers. Still more particularly, the invention relates to arrangements wherein a properly inserted sheet-like X-ray film is flanked by two flexible intensifier screens and the screens are urged against the respective sides of the film by a ferromagnetic layer which is outwardly adjacent to one of the screens and a magnetic plate or layer which is outwardly adjacent to the other screen. The magetic layer attracts the ferromagnetic layer to thereby bias the screens against the respective sides of the X-ray film.
As a rule, sheet-like X-ray films are inserted into cassettes prior to transfer or transport to the exposure station. However, it is also known to withdraw sheet-like X-ray films directly from a stack or another source of supply and to transfer such films, without resort to cassettes, to the exposure station where the films are confined substantially in the same way as in a cassette, i.e., between two intensifier screens which are urged against the respective sides of the film therebetween by magnetic means, i.e., by a ferromagnetic layer and a magnetic layer which latter attracts the ferromagnetic layer and thereby maintains the screens in full face-to-face contact with the respective sides of the film. Once the film is exposed, it is transported immediately to the developing station.
German Pat. No. 1,112,887 to Schlenker discloses a cassette for X-ray films wherein two intensifier screens flank a properly inserted film. One of the screens is flexible and exhibits magnetic properties to cooperate with a layer which consists of iron and is applied directly to the bottom wall of the cassette. A drawback of the patented cassette is that the ferromagnetic layer which is bonded to the bottom wall of the cassette is likely to be scratched or otherwise damaged. As a rule, the ferromagnetic layer is connected to the bottom wall by utilizing a suitable adhesive. The replacement of a damaged ferromagnetic layer is a time-consuming and costly procedure because such layer must be scraped off the inner side of the bottom wall.
Another serious drawback of the patented cassette is that the ferromagnetic layer is likely to become detached from the bottom wall of the cassette in response to pronounced temperature changes. This is due to the fact that the ferromagnetic layer is extremely thin (it must permit X-rays to pass therethrough to the properly inserted film) and that the heat expansion coefficient of its material often deviates (for example, by a factor of up to 10) from the heat expansion coefficient of the material of the bottom wall. Detachment of the ferromagnetic layer can be complete or partial; for example, the partially detached ferromagnetic layer is likely to wrinkle with attendant adverse influence on the quality of exposed X-ray films. Pronounced fluctuations of temperature are not likely to take place in a laboratory where the temperature is or should be maintained within a rather narrow range; however, the difference between the temperatures of air in a well heated laboratory and in a relatively cool (unheated) storage area often suffices to entail partial or complete separation of the ferromagnetic layer from the bottom wall of the cassette. Further opportunities for separation of the ferromagnetic layer from the cassette arise during shipment of cassettes by air (in the unheated cargo area) or by railroad (especially during the winter season). A wrinkled ferromagnetic layer is unsatisfactory not only because its attracting or pressing action is less than necessary but also because a wrinkled ferromagnetic layer is even more sensitive to tearing or other serious damage which necessitates replacement of such layer with a fresh foil of ferromagnetic material.